Sense of Smell Drove Mammal Brains’ Growth

By Yun Xie, Ars Technica

Mammals have the largest brains relative to body size out of all the organisms on Earth. In addition, only mammals have a neocortex, the outer layer of the cerebral hemispheres that participates in higher functions like sensory perception, refined motor coordination, and language. Researchers are interested in figuring out how mammals evolved such large and complex brains. Studying brain evolution has been limited by the rarity of intact skulls and a shortage of techniques that can analyze the interior of those fossils without destroying them.

[partner id=”arstechnica” align=”right”]Mammals evolved roughly 200 million years ago, either before or during the Early Jurassic period. Researchers have access to jaws and teeth from that period, but they rarely come across full skulls. To get information on the brains enclosed by those rare skulls using traditional methods, scientists would have to risk damaging the fossils. Recently, lead researcher Timothy Rowe and his colleagues overcame this problem. They applied high-resolution X-ray computed tomography to nondestructively visualize the interior of ancient skulls. This technique obtains digital information on the 3-D geometries and properties of the inner cavities of solid objects. Their findings appear in a recent issue of Science.

Using high-resolution X-ray computed tomography, Rowe obtained a cast of the interior of a skull of the Morganucodon, a mouse-sized mammal that was around during the late Triassic period, about 205 million years ago. Scientists have proposed that Morganucodon is a link in evolution between mammal-like reptiles and true mammals.

Rowe found that the brain of the Morganucodon would have been nearly 50 percent bigger (relative to body size) than that of earlier Triassic reptiles. This size expansion is mainly driven by the enlargement of the olfactory bulb and olfactory cortex, which are both involved in the sense of smell. The rest of the brain also changed; the cortex and the cerebellum increased in size, making the brain closer in appearance to those of modern mammals than it was to reptiles.

Rowe and his team also used X-ray computed tomography to obtain a digital cast of the interior of a Hadrocodium skull. Hadrocodium, from the Lower Jurassic about 195 million years ago, is a small mammal less than 2 inches long. The Hadrocodium brain is almost another 50 percent bigger (relative to body size) than those of earlier mammals, making it close to the relative brain size of some modern mammals.

Once again, the majority of the brain enlargement came from expansion of the olfactory bulbs and olfactory cortex. The cerebellum also increased in size, which suggests that the synergistic relationship between the sensory system and the motor system improved along with an enhancement in the perception of smell.

Rowe proposes that there would have been another round of enhancement for the olfactory system to obtain the size and level of sophistication of modern mammalian ones. There would have been an evolution of nasal bones, along with an expansion of the olfactory epithelium (epithelial tissue in the nasal cavity that detects odors) by a factor of 10.

Based on these two fossils, Rowe proposes that the evolution of mammalian brain occurred in major stages that all involve significant expansion of the olfactory system. It appears that mammalian brains increased in size along with their ability to detect odors with high resolution.

While Rowe offers concrete evidence of mammalian brain evolution, it would be helpful to see high-resolution X-ray computed tomography studies of other skulls along the path from reptiles to modern mammals. Their methods could also be used to examine the brain evolution of other organisms, such as birds.